The present invention relates to a new and improved construction of a shoe or friction brake which is of the type comprising two separate brake systems. Each brake system consists of a brake lever, a brake jaw or shoe, a brake spring and a brake venting device. The brake levers are connected independently of one another with a stationary brake housing both at their pivotably mounted end and also at the end driven by the brake spring.
Swiss Patent No. 374,413 discloses a shoe or friction brake which can be particularly used for elevators. With that prior art brake system the shoe brake is vented by load measurement and producing a counter torque or moment of rotation of the drive motor, so that the elevator, after brake venting, can start up without any jerky movements. During the load measurement the shoe or friction brake performs a pivotal movement about the center of the brake disk. For this purpose the brake levers are mounted at a beam carrying at both ends rollers guided in arcuate cams. At the centre of the beam there is arranged an actuation arm acting upon the load measuring device. The free ends of the brake levers are interconnected with one another by means of brake springs and a traction rod. The drawback of this shoe brake resides particularly in the fact that, upon rupture of a brake spring both of the brake shoes or jaws become ineffectual. Other drawbacks of this state-of-the-art system reside in the relatively complicated guiding of the brake lever in the arcuate cams and the therewith associated disadvantage that the load measuring device must be capable of absorbing the entire load which is to be measured, and hence, must be correspondingly constructed.
On the other hand, Swiss Patent No. 530,338 discloses a safety shoe brake for elevators, wherein the two brake levers are connected independently of one another with a stationary brake housing both at the pivotably mounted end and also at the end which is driven by the brake spring. At the driven end this connection is obtained by providing for each brake lever a rod secured at one end at the stationary brake housing, and possessing at the other end a support for the brake spring which acts upon the brake lever. Since with this system design there are provided two independently operating brake levers, with this brake system, upon failure of one of the brake shoes or jaws, for instance due to rupture of a brake spring, the second brake shoe or jaw nonetheless remains operative. This safety shoe brake is not suitable for performing the load measurements needed for the jerk-free start-up at elevators. This is so because due to the stationary arrangement of the brake lever-pivot axis and the rod carrying the brake springs, the brake cannot perform any pivotal movement about the center of the brake disk.